The present invention concerns a magnetic memory and it particularly relates to a magnetic domain wall moving type magnetic random access memory.
In recent years, a magnetic random access memory (MRAM) using a ferromagnetic film having a magnetoresistance effect has been proposed as one of types of non-volatile memories and, particularly development has been conducted vigorously for MRAM using a magnetic tunneling junction film having a giant magnetoresistance effect.
The magnetic tunneling junction (MTJ) is based on a stacked structure in which a non-magnetic insulating film (hereinafter referred to as “tunnel barrier film”) is put between a first ferromagnetic film and a second ferromagnetic film. MRAM includes a device using the stacked structure as a memory cell. When current flows in a direction perpendicular to the film surface of the stacked structure, the electric resistance changes depending on the relative angle of the magnetic moment between the first ferromagnetic film and the second ferromagnetic film. The electric resistance is minimized when magnetic moments are in parallel and maximized when they are in anti-parallel to each other. The change of the value of the electric resistance is referred to as a tunneling magneto resistance effect (TMR effect). As the ratio of the TMR effect to the value of the electric resistance (TMR ratio) is greater, it is more advantageous for reading recorded information.
Data is stored in a MRAM by corresponding the case where the direction of the magnetic moments of the two ferromagnetic films are in parallel to binary information “1” and the case where they are in anti-parallel to information “0” respectively. As the TMR ratio is greater, the signal difference between “1” and “0” is larger and reading is easy. Usually, in the two ferromagnetic films, one ferromagnetic film is defined as “pinning layer” (or “reference layer”) where the magnetic moment is fixed and the other ferromagnetic film is defined as “free layer” where the direction of the magnetic moment can be changed. Data is stored by putting the magnetic moment of the free layer in parallel or anti-parallel to the magnetic moment of the pinning layer. In the present specification, the free layer as a ferromagnetic film where data is stored is referred to as “magnetic recording layer”.
“Asteroid mode (for example, refer to U.S. Pat. No. 5,640,343)” and “toggle mode (for example, refer to U.S. Pat. No. 6,545,096 and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-505889 (U.S. Pat. No. 7,184,300) have been known as a method of writing data to MRAM. According to the write systems described above, a reversed magnetic field necessary for reversing the magnetization of a magnetic recording layer increases substantially in inverse proportion to a memory cell size. That is, the write current tends to increase as the memory cell is refined.
As the write method capable of suppressing increase in the write current along with refinement, “a spin transfer method” has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-093488 (U.S. Pat. No. 7,193,284) and K. Yagami et al, “Research Trends in Spin Transfer Magnetization Switching, Journal of the Magnetics Society of Japan Vol. 28, p. 937 (2004)”. According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor and magnetization is reversed by direct internal action between the spin of conductive electrons carrying on the current and magnetic moment of the conductor (Spin Transfer Magnetization Switching). However, when the spin transfer magnetization switching system is applied to the TMR effect device, a current flows in the direction of the film thickness of the magnetic tunnel junction to possibly destroy the tunnel barrier film by the voltage applied to the magnetic tunnel junction.
As a countermeasure, there has been proposed a method of flowing a current in an in-plane direction of a magnetic recording layer, and reversing the magnetization in the magnetic recording layer to a direction in accordance with the direction of the write current due to the effect of the spin transfer (transfer of spin angular amount of movement) by spin electrons (magnetic domain wall moving type) (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-191032, WO 2005-069368 (US Patent Laid-Open No. 2008137405) and Japanese Unexamined Patent Application Publication No. 2006-73930). The write method according to the system is to be explained briefly with reference to FIG. 1.
FIG. 1 is a cross sectional view showing a configuration of a typical magnetoresistance effect device. The magnetoresistance effect device has a magnetic recording layer 110, a reference layer 112, and a tunnel barrier film 111 put between the magnetic recording layer 110 and the reference layer 112. In FIG. 1, axis x and axis y are defined in parallel to in-plane direction of the magnetic recording layer 110 and the axis z is defined in the direction of the film thickness of the magnetic recording layer. The magnetic recording layer 110 includes a first magnetization pinned region 101, a second magnetization pinned region 103, and a magnetic reversal region 102. The magnetic reversal region 102 has a reversible magnetization and overlaps with the reference layer 112. The first magnetization pinning layer 101 is connected with a first boundary 121 of the reversible region 102 and the direction of the magnetization thereof is fixed in the direction +z. The second magnetization pinned region 103 is connected with a second boundary 122 of the magnetization reversal region 102 and the direction of magnetization thereof is pinned in the direction −z (opposing the magnetization of the first magnetization pinned region 101). Magnetizations in the first and second magnetization pinned regions 101 and 103 in the direction anti-parallel with each other.
On the other hand, magnetization of the magnetization reversal region 102 can be reversed in the direction of the film thickness of the magnetic recording layer 110 and directed to one of the direction +z or the direction −z in a stationary state. In the magnetic recording layer 110, the magnetic domain wall (Domain Wall; DW) is formed to one of the first boundary 121 and the secondary boundary 122. The reference layer 112 is formed so as to oppose the magnetization reversal region 102 of the magnetic recording layer 110 with the tunnel barrier film 111 put between them. The reference layer 112, the tunnel barrier film 111, and the magnetization reversal region 102 provide magnetic tunneling junction (MTJ).
In addition, the magnetoresistance effect device includes a first magnetization pinning layer 119 joined to the first magnetization pinned region 101 and a second magnetization pinning layer 120 joined to the second magnetization pinned region 103. The first magnetization pinning layer 119 includes a magnetically hard ferromagnetic material and has a magnetization in the direction +z. In the same manner, the second magnetization pinning layer 120 includes a magnetically hard ferromagnetic material and has a magnetization in the direction −z. The first magnetization pinning layer 119 has a function of fixing the magnetization of the first magnetization pinned region 101 in the direction +z. The second magnetization pinning layer 120 as a function of fixing the magnetization of the second magnetization pinned region 103 in the direction −z.
Data is written to such a magnetoresistance effect device as described below. It is to be explained assuming that the state where magnetization of the magnetization reversal region 102 is directed to the direction −z and the domain wall is positioned at the first boundary 121 corresponds to data “1” and the state where the magnetization of the magnetization reversal region 102 is directed to the direction +z and the domain wall is situated at the second boundary 122 corresponds to data “0”. However, it will be apparent to a person skilled in the art that correspondence between the magnetization direction and the value of data are invertible.
When data “1” is written to the magnetic recording layer 110 where data “0” has been written, a write current is supplied from the first magnetization pinned region 101 through the magnetization reversal region 102 to the second magnetization pinning layer 103. That is, a spin-polarized current is transferred from the second magnetization pinned region 103 to the magnetization reversal region 102. Thus, the domain wall moves from the second boundary 122 to the first boundary 121 and magnetization of the magnetization reversal region 102 is directed to the direction −z, that is, data “1” is written. On the other hand, when data “0” is written into the magnetic recording layer 100 where the data “1” has been written, a write current is supplied from the second magnetization pinned region 103 through the magnetization reversal region 102 to the first magnetization pinned region 101. That is, the spin polarized electrons are transferred from the first magnetization pinned region 101 to the magnetization reversal region 102. Thus, the magnetic domain wall moves from the first boundary 121 to the second boundary 122 and the magnetization of the magnetization reversal region 102 is directed to the direction +z, that is, data “0” is written. As described above, data is written when the magnetic domain wall (DW) in the magnetic recording layer 110 moves between the first boundary 121 and the second boundary 122 of the magnetization reversal region 102 by the current flowing between the first magnetization pinned region 101 and the second magnetization pinned region 103.
According to this mode, since current flowing upon writing does not pass through the tunneling barrier film 111, deterioration of the tunneling barrier film 111 is suppressed. Further, since data is written by the spin transfer mode, the write current is decreased along with reduction of the memory size. Further, since the moving distance of the magnetic domain wall (DW) is decreased as the memory size is reduced, the write speed increases along with refinement of the memory cell.
FIG. 1 shows a case where the magnetic recording layer 110 has a perpendicular magnetic anisotropy and magnetization of the magnetic recording layer 110 is directed to the direction of the film thickness. However, the magnetization of the magnetic recording layer may also be directed to the in-plane direction. The configuration where the magnetization of the magnetic recording layer is directed to the in-plane direction is disclosed, for example, in A. Yamaguchi et al., “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, Physical Review Letters, Vo. 92. p. 077205 (2004). However, for decreasing the write current, a configuration where the magnetization of the magnetic recording layer is directed to the direction of the film thickness is more preferred than the configuration where the magnetization of the magnetic recording layer is directed to the in-plane direction. In the technique disclosed by A. Yamaguchi et al. described above, a current density necessary for moving a current-induced magnetic domain wall is about 1×108 [A/cm2]. In this case, the write current is 1 mA for the magnetic recording layer 110, for example, of 100 nm width and 10 nm film thickness. On the other hand, as described in S. Fukami et al., “Micromagnetic analysis of current driven domain wall motion in nanostrips with perpendicular magnetic anisotropy”, Journal of Applied Physics, vol. 103, p. 07E718 (2008), it is reported that the write current can be decreased sufficiently by using a material having a perpendicular magnetic anisotropy as the magnetic recording layer. In view of the above, in a case of manufacturing a MRAM by utilizing the current-induced magnetic domain wall motion, it can be said that a ferromagnetic material having perpendicular magnetic anisotropy is used preferably for the layer of causing magnetic domain wall motion. H. Tanigawa et al, reported in “Current-Driven Domain Wall Motion in CoCrPt Wires with Perpendicular Magnetic Anisotropy” Applied Physics Express, vol. 1, p. 011301 (2008) that a current-induced magnetic domain wall motion was observed in a material having perpendicular magnetic anisotropy. As described above, it is expected that a MRAM decreased in the write current is provided by utilizing the phenomenon of the current induced magnetic domain wall motion in the material having perpendicular magnetic anisotropy.
As a relevant technique, WO 2009/001706 (US Patent Laid-Open No. 2010188890) discloses a magnetoresistance effect device and a magnetic random access memory. The magnetoresistance effect device includes a magnetization free layer, a spacer layer disposed adjacent with the magnetization free layer, a first magnetization pinning layer disposed adjacent with the spacer layer on the side opposite to the magnetization free layer, and at least two magnetization pinning layers disposed adjacent with the magnetization free layer. The magnetization free layer, the first magnetization pinning layer, and the second magnetization pinning layer have a magnetization component substantially perpendicular to the film surface. The magnetization free layer has two magnetization pinned portions and a magnetic domain wall moving portion disposed between the two magnetization pinned portions. Magnetizations of the two magnetization pinned portions forming the magnetization free layer are fixed substantially in anti-parallel to each other in the direction substantially perpendicular to the film surface. The magnetic domain wall moving portion is provided with magnetic anisotropy in the direction perpendicular to film surface.
Further, Japanese Patent Application Publication No. 2009-182129 (US Patent Laid-Open No. 2009190262) discloses a magnetoresistance effect device and a manufacturing method thereof. The magnetoresistance effect device has a magnetoresistance effect film and a pair of electrodes for flowing a current perpendicular to the film surface of the magnetoresistance effect film. The magnetoresistance film includes a magnetization pinning layer, a magnetization free layer, an intermediate layer, a cap layer, and a functional layer. In the magnetization pinning layer, the magnetization direction is fixed substantially in one direction. The magnetization direction of the magnetization free layer changes in accordance with an external magnetic field. The intermediate layer is disposed between the magnetization pinning layer and the magnetization free layer. The cap layer is disposed above the magnetization pinning layer or the magnetization free layer. The functional layer is disposed in the magnetization pinning layer, in the magnetization free layer, at the boundary between the magnetization pinning layer and the intermediate layer, at the boundary between the intermediate layer and the magnetization free layer, or the boundary between the magnetization pinning layer or the magnetization free layer and the cap layer and is formed of a material containing oxygen or nitrogen. The crystal orientation face of the functional layer is different from the crystal orientation face of the adjacent layer above or below thereof.